In each currently prevailing LCD device such as a TV set, two pieces of high-precision glass sheets are used, one as a substrate for the electronic circuit components such as thin film transistors (TFT), and the other as a substrate for a color filter. Various technologies are available for making such high-precision optical glass substrates. A leading one is the overflow fusion down-draw technology developed by Corning Incorporated, Corning, N.Y., U.S.A., which involves the use of a forming body typically called an isopipe, a forming pipe, a forming trough, or a forming device. The forming body typically comprises an upper part comprising two upper trough walls and a trough bottom defining a trough space for accepting molten glass, and a lower part having a wedge-shaped cross-section having two major side surfaces sloping downwardly to join at a root, where the side walls of the upper part and the lower part form two continuous forming surfaces. During operation, molten glass is filled into the trough, allowed to overflow the top surfaces (called weirs) of the trough, down along the two major side surfaces, and then join at the converging line of the two surfaces (called root) to form a unitary glass ribbon with both external surfaces that have not been exposed to the surface of the forming body. The ribbon is drawn down and cooled to form an elastic glass sheet having desired thickness and pristine external surfaces.
The manufacture of high quality LCDs having high image quality with ever growing sizes imposes ever more stringent requirements on the geometric and dimensional stability of the forming body. In order to make continuous glass sheet with sufficient width, the forming body is typically suspended on the ends only. During operation, the forming body is subjected to the gravity of its own and the molten glass it supports, and very high operating temperature such as 1200-1300° C. At such high temperature and large external load, it is very challenging to address the stability issue over a long manufacture campaign. Corning Incorporated has successfully developed various technologies over the past few decades to solve this problem, including the application of upward bending moment to the forming body by imposing compressive forces at both the inlet and the compressive ends, and the use of high-performance materials that are less subjected to sagging.
However, as the desirable size of the glass sheet grew larger and larger over time, so was the horizontal span of the forming body. A forming body with a larger span (length) made of the same material tends to distort more in a given period of time and at a given working condition. In addition, a unitary forming body made of a single material meeting all geometric and shape stability requirements is very expensive, even at relatively small size.
Therefore, there is a genuine need of a forming body design for the overflow fusion down-draw process which is economical to fabricate, maintain, operate and suitable for making glass sheets having a large width.
The present invention satisfies this and other needs.